DESERT REGIONS OF NORTH AMERICA. 25 
also a Euphorbia and a Pedilanthus with thick upright cylindrical stems, 
in which the storage function is made more effective by the possession 
of a thick milky juice. | 
The tree morning-glory (Jpomea) has a soft, thick trunk, into which a 
knife may be easily thrust to the hilt, the medullary tissues being highly 
charged with water and containing some reserve food material. 
Perhaps of all of the plants which show this capacity, however, 
Beaucarnea edipus (plate 19) is the most remarkable. This relative of 
the Yucca, like all plants of this group with narrow leaves, is known as 
‘“‘sotol’’ and has the bases of the trunks swollen in adult specimens to a 
diameter of 7 or 8 feet, the topmost branch not reaching a height of 
more than 25. This trunk has a truncate base resting almost upon the 
top of the ground, to which it is attached by a few slenderroots. This 
storage organ is composed of a mass of parenchymatous tissue through 
which run irregularly strands of fibrovascular bundles. After death the 
loss of water reduces the weight of the storage organ so much that a large 
plant may be easily toppled over as it stands. 
From experimental cultures under way at the Desert Laboratory, the 
roots appear to have their originin some deeply internal layer and to push 
their way forcibly through the thick mass of the storage tissue until the 
soilis reached. The peculiar form of the stem is predicated by its devel- 
opment in the first three months of the growth of the seedling. The 
sap has a very bitter taste and it could not be found that any animal 
makes use ofit, no matter how badly in need of water. 
Two species of grape were found (Czssus) in which enlargements of 
the climbing stems occurred at the bases or at various heights from the 
ground, making globoid tubers as much as 5 or 6 inches in diameter, 
which served as efficient storage and propagative organs. The storage 
function in this plant is taken on very early in the history of the sporo- 
phyte, as was found by the cultivation of seedlings in the Desert Labora- 
tory. In these experiments the hypocotyl and the first internode of the 
stem were seen to undergo a thickening even when the plantlet bore but 
one or two leaves and the resulting tuber eventually reached a thick- 
ness of 1 to 2 inches. 
No reliable records are at hand, but if the character of the vegetation 
may be relied upon it may be assumed that the precipitation of the 
Tehuacan region comes within a brief period during the year, during 
which time a reserve supply may be stored up by plants. By reference 
to the data given for Puebla, it is to be seen that five-sixths of the 
rainfall occurs within five months, and five-sevenths within four months, 
at Puebla, and the general aspect of the vegetation of Tehuacan indicates 
a shorter rainy season. 
The epiphytic habit was prominently displayed by a number of species, 
including the opuntias and bromeliads. Prickly-pears were seen growing 
